Method of pickling titanium and titanium alloys



United States Patent METHOD OF PICKLING TITANIUM AND TITANIUM ALLOYS George Francis Alhers and William Budd Stephenson, Jr., Cincinnati, Ohio, assignors to General Electric C01 pany, a corporation of New York No Drawing. Application October 24, 1955 Serial No. 542,472

6 Claims. (Cl. 134-28) This invention relates to a method of pickling and, more particularly, to a method of pickling and removing scale from titanium and titanium base alloys.

Considerable difiiculty has been encountered in removing scale from alloys of the above type. General practice has been to pickle titanium in a hot nitric-hydrofluoric acid bath (l0%2%). However, it has been found that due to an unequal rate of attack on the scale, pickling in this bath quite often leaves the base metal rough and pitted.

It is desirable to form sheet titanium at elevated temperatures due to the fact that it has a tendency to crack when formed in the cold state. Temperatures used for forming usually vary between 450 F. to 1000 F. according to the severity of the forming operation. After forming the parts are covered with a scale. The character of the scale varies with the forming temperature. The scale interferes with subsequent forming and welding operations and must be removed.

it is an object of this invention to provide a method of removing scale from titanium and titanium alloys.

Still another object of this invention is to provide a method of pickling titanium and titanium alloys wherein the base metal is not unduly attacked by the pickling solution.

Briefly stated, in accordance with one aspect of our invention, we provide a method of pickling titanium or titanium alloy articles by immersing them in an aqueous solution of sulfuric acid and calcium fluoride and thereafter immersing the articles in a second cleaning bath such as an aqueous solution of a mineral acid.

After considerable experimental work wherein various pickling solutions were tested, we discovered that an aqueous solution containing sulfuric acid and calcium fluoride would produce exceptionally improved results. We determined that an article of titanium or a titanium alloy should be immersed in the above mentioned solution prior to a short treatment in a cleaning bath, for example a mineral acid aqueous solution such as a mixture containing nitric and hydrofluoric acids. We found that the sulfuric acid plus calcium fluoride aqueous solution converted the scale to a gray film that had low surface resistance which would be easily removed by a short immersion in a mineral acid bath. The surface was left smooth and bright. X-ray diffraction studies of the gray film did not reveal its exact nature or composition.

We found that a satisfactory aqueous solution for converting the scale to a gray film on the titanium or titanium alloy was made up as follows:

Water Concentrated sulfuric acid ml. 65 Calcium fluoride -grams 2,827,402 Patented Mar. 18, 1958 operation. An insoluble precipitate of calcium sulfate will be slowly formed. The sulfuric acid content can be controlled by simple titration. Infrequent analysis of the suspended material in the bath will insure a continuing excess of calcium fluoride. Analysis has shown that only six grams per litre of solution will insure an excess of calcium fluoride. However, using smaller amounts of calcium fluoride in making up the bath will necessitate more frequent analysis during operation of the bath.

The sulfuric acid-calcium fluoride aqueous solution is held at a temperature between 150 F. and 170 F. for best results. We have found by experiment, however, that satisfactory results may also be obtained by using any temperature between room temperature and 212 F. The titanium or titanium alloy article is immersed in the sulfuric acid-calcium fluoride aqueous solution for a period of from 5 to 10 minutes which has been found to be sufficient in order to form the aforementioned gray film. The time may vary with the temperature of the solution used. The article immersed in the sulfuric acid-calcium fluoride solution upon removal is rinsed in order to prevent contamination of the mineral acid solution in which the article is to be subsequently immersed.

The titanium or titanium alloy article is next immersed in a cleaning bath, an example of which is a mineral acid bath such as hydrochloric acid, hydrofluoric acid, nitric acid or mixtures thereof. Good results, however, have been obtained by using an aqueous solution of 10% by weight nitric acid and 2% by weight hydrofluoric acid with the balance water. The nitric-hydrofluoric acid is maintained at approximately 160 F. for good results. The titanium or titanium alloy article is then immersed in the nitric-hydrofluoric acid aqueous solution for a period of one to two minutes which removes the gray film previously formed in the sulfuric acid-calcium fluoride aqueous solution. Upon removal from the nitric-hydrofluoric acid aqueous solution, the titanium or titanium alloy article is completely free of any scale that would interfere with subsequent welding or forming operations.

After removal from the cleaning bath, the article may be rinsed and blown dry in order to remove the cleaning bath film from the surface of the article. Any surface smut still remaining may be removed by spraying with water. I

While a particular embodiment of the invention has been described, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the invention and it is intended to cover in the appended claims all such changes and modifications that come within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A method of the character described for removing scale from a titanium or titanium alloy article comprising the steps of immersing the article in an aqueous solution consisting of sulfuric acid and calcium fluoride until the scale is converted to a gray film thereafter immersing the article in a mineral acid solution to remove the gray film.

2. A method as defined in claim 1 wherein the mineral acid solution consists of an aqueous solution of nitric acid and hydrofluoric acid.

3. A method as defined in claim 1 which comprises rinsing the article in water after removal from the sulfuric acid and calcium fluoride aqueous solution.

4. A method as defined in claim 1 wherein the sulfuric acid and calcium fluoride aqueous solution is maintained at a temperature of about to F.

5. A method as defined in claim 1 wherein the sulfuric acid and calcium fluoride aqueous solution has a temperature of about 150 to 170 F., and the articles are subiectedio fliesnhlflbnlf oizamaverage tinie 6f aboutS-to 10 minutes. 7 V

6. A method of the ehatacter deseribed for remeving scale from a titanium and titanium alloy articlecomprisingkther steps offirmnersi-n'gztherertileinan agitateiaquemls perature between mom tempenature'..andi2l2 F", Iinsing .snlmioneoiaqnfinaralamd.

References Cited inthe file of this patent UNITED STATESVPATENTS MacPhersdn Nov. 22, 1955 

1. A METHOD OF THE CHARACTER DESCRIBED FOR REMOVING SCALE FROM A TITANIUM OR TITANIUM ALLOY ARTICLE COMPRISING THE STEPS OF IMMERISING THE ARTICLE IN AN AQUEOUS SOLUTION CONSISTING OF SULFURIC ACID AND CALCIUM FLUORIDE UNTIL THE SCALE IS CONVERTED TO A GRAY FILM THEREAFTER IMMERSING THE ARTICLE IN A MINERAL ACID SOLUTION TO REMOVE THE GRAY FILM. 